It has long been known that silica-based optical fibers (i.e., optical fibers having cladding and core compositions which are at least 80% silicon dioxide) may be chemically attacked by environmental water, resulting in weakening of the fibers. In experimental tests, evidence of such weakening may be provided by static fatigue tests, in which optical fibers are subjected to constant stress for an extended period of time in an active environment such as hot water. The resistance of a particular class of fibers to corrosion may be related to the fatigue life, i.e., the average time which elapses before the fibers fail in such tests.
Practitioners have tried various methods for improving the fatigue life of optical fibers. For example, U.S. Pat. No. 5,033,815, issued to T. Edahiro, et al. on Jul. 23, 1991, describes efforts to improve the fatigue life of optical fibers by modifying the chemical composition of an outer glass cladding layer. An alternative technique is to use a hermetic coating to prevent water from coming into contact with the glass of the optical fiber. Hermetic coatings of carbon are described, e.g., in U.S. Pat. No. 5,000,541, issued to F. V. DiMarcello, et al. on Mar. 19, 1991.
It is conventional to enclose optical fibers within protective, polymeric jackets. If a polymeric jacket could be provided which gave effective protection against water, such a jacket would be advantageous either as a supplement, or as an alternative, to the measures described above. In particular, such a jacket could potentially be used independently of the particular fiber design and composition, and could be used to protect relatively simple and inexpensive fibers. However, although polymeric jackets have been found useful for maintaining the mechanical and optical properties of optical fibers, they have also generally been found to be permeable to water. As a consequence, they are only of limited value for protecting fibers from weakening by aqueous attack.